Traditional spinal stabilization systems include the type which has multiple bone anchors for attaching to the respective vertebrae, and a linking member secured to the bone anchors to prevent or limit the motion between the vertebrae. Anchors typically include pedicle screws and/or hooks. Linking members typically include rods and/or plates. Alternatively, the linking member may be a substantially flexible cord, which permits relative motion between the stabilized vertebrae, and a resilient spacer between each pair of linked bone anchors. One exemplary system is made of braided Polyethylene Terephthalate (PET; available under the Sulene™ brand from Sulzer Orthopedics, Ltd., Baar, Switzerland). An exemplary spacer is made of Polycarbonate Urethane (PCU, also available under the Sulene™ brand name from Sulzer Orthopedics, Ltd.).
In the prior exemplary systems, the spacer generally operates to resist, without completely preventing, motion of the bone anchors toward each other, while the cord operates to resist motion of the bone anchors away from each other. Because the spacer and cord are generally more flexible than rigid metal rods or plates, systems employing a cord and/or spacer arrangement generally permit movement between vertebrae and thus provide a more dynamic stabilization system than those employing rigid linking members. Additionally, the interconnections between the anchors, spacers, and cords may permit relative motions that are meant to be prevented with rigid metal systems, which also provide a more dynamic stabilization system than those employing rigid linking members.
Each of the foregoing types of systems may have advantages and disadvantages in a given patient condition. Because of the diverse patient conditions requiring treatment, however, there is a need in the art for a spinal stabilization system that combines the advantages of rigid and dynamic stabilization systems, and permits the physician to modify the rigidity of the system based on the needs of the patient.